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Description/Abstract

Planktic foraminifera have been used as recorders of the neodymium (Nd) isotopic composition ofseawater, although there is still controversy over the precise provenance of the Nd signal. We present anextensive, multispecific plankton tow Nd/Ca data set from several geographic locations (SE Atlantic, NEAtlantic, Norwegian Sea, and western Mediterranean), together with core top samples from theMediterranean region. The range of Nd/Ca ratios in plankton-towed foraminifera, cleaned only of organicmaterial, from all regions (0.01–0.7 umol/mol), is similar to previously published analyses of sedimentaryforaminifera cleaned using both oxidative and reductive steps, with distribution coefficients (Kd) rangingbetween 4 and 302. For the Mediterranean, where core top and plankton tow data are both available, therange for plankton tows (0.05–0.7 umol/mol) is essentially identical to that for the core tops (0.1–0.5 umol/mol). Readsorption of Nd during cleaning is ruled out by the fact that the plankton tow samplesunderwent only an oxidative cleaning process. We find a relationship between manganese (Mn) and Nd inplankton tow samples that is mirrored by a similar correlation in core top samples. This relationshipsuggests that Fe-Mn coatings are of negligible importance to the Nd budgets of foraminifera as the Nd/Mnratio it implies is over an order of magnitude greater than that seen in other Fe-Mn oxide phases. Rather,since both plankton tows and core tops present a similar behavior, the Nd/Mn relationship must originate inthe upper water column. The data are consistent with the acquisition of Nd and Mn from the water columnby binding to organic material and the fact that intratest organic material is shielded from both aggressivecleaning and diagenetic processes. Collectively, the results help to explain two abiding puzzles about Nd insedimentary planktic foraminifera: their high REE contents and the fact that they record a surface water Ndisotopic signal, regardless of the cleaning procedure used.